Link adaption technique, or adaptive modulation and coding (AMC) technique, is widely used in wireless communication systems to increase the reliability and throughout of data transmission. In wireless communication systems, link adaption refers to automatically selecting, based on transmission channel detection, suitable transmission configuration parameters for a transmission link, such as modulation and coding scheme (MCS) and transmission power, so as to adapt to a channel varying real-time. Channel quality detection is particularly important to the link adaption technique, and its accuracy directly influences the performance of the whole system. Herein, a link refers to a wireless communication path having a certain bandwidth and consisting of a plurality of sub-carriers. In the context of the present description, the terms “channel” and “link” have the same meaning. However, in view of the expression habits of those skilled in the art, the terms “channel” and “link” may be respectively used at different scenarios.
In a narrow-band wireless communication system, average quality of a plurality of sub-carriers in a channel, i.e., average channel quality, is used as a basis for selecting transmission configuration parameters for a transmitting part. Usually, a physical signal to interference and noise ratio (PSINR) or an average signal to interference and noise ratio is used as an indicator for measuring channel quality. Such a method has an advantage of simple computation.
In an broadband wireless communication system, such as an orthogonal frequency division multiplexing (OFDM) system, an effective SINR (ESINR) or an effective signal to noise ratio (ESNR) of a channel is often used as an quality indicator of the channel (also referred to as an effective quality indicator of the channel) for overcoming influence from frequency selectivity generated due to multipath.
FIG. 1 is a block diagram of a typical closed loop OFDM system using the link adaption technique. The OFDM system includes a transmitting part and a receiving part. The transmitting part as a transmitter includes an antenna, a radio frequency (RF) unit 105, an inverse fast Fourier transformer (IFFT) 104, a sub-carrier mapping unit 103, a modulator 102, a channel encoder 101 and a transmitter controller 106. The receiving part as a receiver includes an antenna, an RF unit 107, a fast Fourier transformer (FFT) 108, a sub-carrier demapping unit 109, a channel estimating unit 113, an equalizing unit 110, a demodulator 111, a channel decoder 112 and a receiver selector 114.
As shown in FIG. 1, at the transmitting part, the transmitter controller 106 configures transmission parameters such as a coding and modulation scheme for a link. The coding and modulation scheme is a combination of a coding scheme (CS) and a modulation mode (MD). A data stream to be transmitted, after being processed by the configured channel encoder 101 and modulator 102, the sub-carrier mapping unit 103 and the IFFT 104, is transmitted out through the antenna after being processed by the RF unit 105. At the receiving part, the RF unit 107 performs RF processing on a received signal transmitted through a radio channel, and sends the obtained baseband digital signal to the FFT 108. On one hand, the data output from the FFT 108 is transmitted to the sub-carrier demapping unit 109 for sub-carrier demapping, and, on the other hand, is also transmitted to the channel estimating unit 113 for channel estimation. After the equalizing unit 110 equalizes a signal output from the sub-carrier demapping unit 109 by using a channel estimation value output from the channel estimating unit 113, the equalized signal is demodulated and decoded by the demodulator 111 and the channel decoder 112 respectively. The channel estimation value output by the channel estimating unit 113 is also sent to the receiver selector 114 for calculating the effective quality indicator such as the ESINR or the ESNR of a channel and selecting the parameters for the next transmission. The selection result is fed back to the transmitter controller 106.
Here, the ESINR is a combination of SINRs of sub-carriers in an input signal of the receiving part. In practical computation, the SINRs of the sub-carriers refer to processed signal to interference and noise ratios of the sub-carriers (also referred to as instantaneous SINRs), which constitute an instantaneous SINR vector. If γn (n=1:N) denotes an instantaneous SINR obtained based on the estimation channel value, where N is the number of effective sub-carriers used in the system, then the ESINR γeff of the channel can be represented as:γeff=f(γ1,γ2, . . . ,γN)  (1)
Usually, the mapping process of equation (1) is referred to as effective SINR mapping (ESM). Traditional ESM methods mainly include mutual information-ESM (MI-ESM), exponential ESM (EESM) and capacity ESM (CESM), etc. All of them can be represented by an equation (2) as follows:
                              γ          eff                =                              α            1                    ⁢                                    Φ                              -                1                                      ⁡                          [                                                1                  N                                ⁢                                                      ∑                                          n                      =                      1                                        N                                    ⁢                                      Φ                    ⁡                                          (                                                                        γ                          n                                                                          α                          2                                                                    )                                                                                  ]                                                          (        2        )            wherein α1 and α2 are parameters related to the modulation and coding scheme as being used, and Φ(*) is an invertible mapping function.
Different ESM methods have different mapping functions. The following equations (3)-(5) show mapping functions Φ(*) of the above-mentioned traditional ESM methods.
I. Mapping Function of MI-ESM:
                              Φ          ⁡                      (                          γ              n                        )                          =                                            log              2                        ⁡                          (              M              )                                -                                    1              M                        ⁢                                          ∑                                  m                  =                  1                                M                            ⁢                                                E                  U                                ⁢                                  {                                                            log                      2                                        ⁡                                          [                                              1                        +                                                                              ∑                                                                                          k                                =                                1                                                            ,                                                                                                                          ⁢                                                              k                                ≠                                m                                                                                      M                                                    ⁢                                                      exp                            ⁡                                                          (                                                              -                                                                                                                                                                                                                                                                                                                  X                                            k                                                                                    -                                                                                      X                                            m                                                                                    +                                          U                                                                                                                                                            2                                                                        -                                                                                                                                                          U                                                                                                                    2                                                                                                                                            1                                    /                                                                          γ                                      n                                                                                                                                                                  )                                                                                                                          ]                                                        )                                                                                        (        3        )            
II. Mapping Function of EESM:
                              Φ          ⁡                      (                          γ              n                        )                          =                  exp          (                      -                                          γ                n                            β                                )                                    (        4        )            
III. Mapping Function of CESM:
                              Φ          ⁡                      (                          γ              n                        )                          =                              log            2                    (                      1            +                                          γ                n                            β                                )                                    (        5        )            
Relevant description of the equation (3) is recorded in an International patent application WO 2006/046894, for example. Relevant description of the equation (4) is recorded in an International patent application WO 2004/098119, for example. Relevant description of the equation (5) may be referred to for in, for example, J. Kim et al., “On Efficient Link Error Prediction based on Convex Metrics”, Proc. IEEE VTC, pages 4190-4194, September 2004. In the three ESM methods, the MI-ESM has a higher accuracy than those of the other two ESM methods.
It can be seen from the above that the mapping function used in traditional ESM methods is usually a nonlinear function. Moreover, in the computation process of the ESINR, SINRs γn of the sub-carriers in a channel are mapped through the Φ(*) and then compressed, and then are mapped through an inverse function of the Φ(*) to become ESINR γeff of the channel. Therefore, the real-time signal processing of the above ESM methods has a relatively high complexity. In addition, M and 13 in the above three equations are all parameters related to the currently used modulation and coding scheme, further improving the calculation complexity.